Redox flow secondary battery and electrolyte membrane for redox flow secondary battery

ABSTRACT

The purpose of the present invention is to provide a redox flow secondary battery which has low electrical resistance and excellent current efficiency in addition to durability. The present invention relates to: an electrolyte membrane for redox flow secondary batteries, which contains an ion exchange resin composition containing a fluorine-based polymer electrolyte; and a redox flow secondary battery which uses the electrolyte membrane for redox flow secondary batteries.

TECHNICAL FIELD

The present invention relates to a redox flow secondary battery, and an electrolyte membrane for a redox flow secondary battery.

BACKGROUND ART

Redox flow secondary batteries are to store and discharge electricity, and belong to large-size stationary batteries used for leveling the amounts of electricity used. The redox flow secondary battery is configured such that a positive electrode and an electrolyte solution containing a positive electrode active substance (positive electrode cell) and a negative electrode and a negative electrode electrolyte solution containing a negative electrode active substance (negative electrode cell) are separated by a separation membrane; charge and discharge are carried out by utilizing the oxidation and reduction reactions of both the active substances; and the electrolyte solutions including both the active substances are circulated from storage tanks to an electrolytic bath, and a current is taken out and utilized.

As an active substance contained in an electrolyte solution, there are used, for example, iron-chromium-based ones, chromium-bromine-based ones, zinc-bromine-based ones, and vanadium-based ones utilizing the difference in electric charge.

Particularly, vanadium-type secondary batteries, since having advantages of a high electromotive force, a high electrode reaction rate of vanadium ions, only a small amount of hydrogen generated as a side-reaction, a high output, and the like, are being developed earnestly.

For separation membranes, devices are made so that electrolyte solutions containing active substances of both electrodes are not mixed. However, conventional separation membranes are liable to be oxidized and for example a problem thereof is that the electric resistance needs to be made sufficiently low. Although in order to raise the current efficiency of batteries, the permeation of each active substance ion contained in the cell electrolyte solutions of both the electrodes (contamination with electrolytes in electrolyte solutions of both electrodes) is demanded to be prevented as much as possible, an ion-exchange membrane excellent in the ion permselectivity, in which protons (H⁺) carrying the charge easily sufficiently permeate, is demanded.

The vanadium-type secondary battery utilizes an oxidation and reduction reaction of divalent vanadium (V²⁺)/trivalent vanadium (V³⁺) in a negative electrode cell, and oxidation and reduction reaction of tetravalent vanadium (V⁴⁺)/pentavalent vanadium (V⁵⁺) in a positive electrode cell. Therefore, since electrolyte solutions of the positive electrode cell and the negative electrode cell contain ion species of the same metal, even if the electrolyte solutions are permeated through a separation membrane and mixed, the ion species are normally reproduced by charging; therefore, there hardly arises a large problem as compared with other metal species. However, since active substances becoming useless increase and the current efficiency decreases, it is preferable that the active substance ions freely permeate as little as possible.

There are conventionally batteries utilizing various types of separation membranes (hereinafter, also referred to as “electrolyte membrane” or simply “membrane”); and for example, batteries are reported which use porous membranes allowing free permeation by an ionic differential pressure and an osmotic pressure of electrolyte solutions as the driving force. For example, Patent Literature 1 discloses a polytetrafluoroethylene (hereinafter, also referred to as “PTFE”) porous membrane, a polyolefin (hereinafter, also referred to as “PO”)-based porous membrane, a PO-based nonwoven fabric, and the like as a separation membrane for a redox battery.

Patent Literature 2 discloses a composite membrane in combination of a porous membrane and a hydrous polymer for the purpose of the improvement of the charge and discharge energy efficiency of a redox flow secondary battery and the improvement of the mechanical strength of a separation membrane thereof.

Patent Literature 3 discloses the utilization of a membrane of a cellulose or an ethylene-vinyl alcohol copolymer as a nonporous hydrophilic polymer membrane excellent in the ion permeability and having a hydrophilic hydroxyl group for the purpose of the improvement of the charge and discharge energy efficiency of a redox flow secondary battery.

Patent Literature 4 states that the utilization of a polysulfone-based membrane (anion-exchange membrane) as a hydrocarbon-based ion-exchange resin makes the current efficiency of a vanadium redox secondary battery 80% to 88.5% and the radical oxidation resistance excellent.

Patent Literature 5 discloses a method of raising the reaction efficiency by making expensive platinum to be carried on a porous carbon of a positive electrode in order to raise the current efficiency of a redox flow secondary battery, and describes a Nafion (registered trademark) N117 made by Du Pont K.K. and a polysulfone-based ion-exchange membrane as a separation membrane in Examples.

Patent Literature 6 discloses an iron-chromium-type redox flow battery in which a hydrophilic resin is coated on pores of a porous membrane of a polypropylene (hereinafter, also referred to as “PP”) or the like. An Example of the Patent Literature uses a membrane covered in a thickness of several micrometers with a fluorine-based ion-exchange resin (made by Du Pont K.K., registered trademark: Nafion) on both surfaces of a PP porous membrane of 100 μm in thickness. Here, Nafion is a copolymer containing a repeating unit represented by —(CF₂—CF₂)— and a repeating unit represented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— wherein X═CF₃, n=1, and m=2.

Patent Literature 7 discloses an example of a vanadium-type redox flow secondary battery decreased in the cell electric resistance as much as possible and raised in the efficiency by the improvement of the electrode sides including the usage of a two-layer liquid-permeable porous carbon electrode having a specific lattice plane.

Patent Literature 8 discloses an example of a vanadium-type redox flow battery using an anion-exchange type separation membrane having a low membrane resistance, being excellent in the proton permeability and the like, and being composed of a crosslinked polymer having a pyridinium group (utilizing N⁺ as a cation). The crosslinked polymer disclosed is a polymer obtained by copolymerizing a pyridinium group-containing vinyl polymerizable monomer, a styrene-based monomer and the like, and a crosslinking agent such as divinylbenzene.

Patent Literature 9 discloses a redox flow secondary battery using a separation membrane which is made by alternately laminating a cation-exchange membrane (fluorine-based polymer or another hydrocarbon-based polymer) and an anion-exchange membrane (polysulfone-based polymer or the like), and which has a cation-exchange membrane disposed on the side of the separation membrane contacting with a positive electrode electrolyte solution, for the purpose of reducing the cell resistance and improving the power efficiency and the like.

Patent Literature 10 discloses a secondary battery using as a separation membrane a membrane excellent in the chemical resistance, low in the resistance, and excellent in the ion permselectivity, which is an anion-exchange membrane made by compositing a porous base material composed of a porous PTFE-based resin with a crosslinked polymer having a repeating unit of a vinyl heterocyclic compound having two or more hydrophilic groups (vinylpyrrolidone having an amino group, or the like). The principle described therein is that although metal cations, having a large ion diameter and a much amount of electric charge, receive an electric repulsion by cations of a separation membrane surface layer part and are inhibited from the membrane permeation under the potential difference application, protons (H⁺), having a small ion diameter and being monovalent can easily diffuse and permeate in the separation membrane having cations to thereby give a low electric resistance.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 2005-158383 -   Patent Literature 2: Japanese Patent Publication No. H6-105615 -   Patent Literature 3: Japanese Patent Laid-Open No. 562-226580 -   Patent Literature 4: Japanese Patent Laid-Open No. H6-188005 -   Patent Literature 5: Japanese Patent Laid-Open No. H5-242905 -   Patent Literature 6: Japanese Patent Laid-Open No. H6-260183 -   Patent Literature 7: Japanese Patent Laid-Open No. H9-92321 -   Patent Literature 8: Japanese Patent Laid-Open No. H10-208767 -   Patent Literature 9: Japanese Patent Laid-Open No. H11-260390 -   Patent Literature 10: Japanese Patent Laid-Open No. 2000-235849

SUMMARY OF INVENTION Technical Problem

However, the battery disclosed in Patent Literature 1 is not sufficient in the electric resistance and the ion permselectivity of the separation membrane, and is insufficient in the current efficiency, the durability, and the like.

The composite membrane disclosed in Patent Literature 2 has a high electric resistance, and a problem thereof is that each ion freely diffuses though not so easily as in porous membranes to thereby give a poor battery current efficiency. The membrane disclosed in Patent Literature 3 also has the similar problem as in the above, and is inferior also in the oxidation-resistant durability.

The battery disclosed in Patent Literature 4 is yet insufficient in the current efficiency, inferior also in the oxidative deterioration resistance in a sulfuric acid electrolyte solution over a long period, and insufficient also in the durability. The Patent Literature, although disclosing a battery using a PTFE-based ion-exchange membrane as a comparative example, states that the current efficiency is 64.8 to 78.6 and insufficient.

The battery disclosed in Patent Literature 5 also cannot solve the similar problem as in the above, and a problem thereof is that the large-size facility is resultantly high in price.

Patent Literature 6 states that the membrane disclosed therein increases in the internal resistance unless the thickness of a coated membrane is made extremely thin (several micrometers). No devices to improve the ion permselectivity are described at all.

The battery disclosed in Patent Literature 7, since using a polysulfone-based separation membrane, is not sufficient in the ion permselectivity and the oxidative deterioration resistance of the separation membrane, and is not sufficient in the electric resistance, the current efficiency, and the durability of the battery.

The battery disclosed in Patent Literature 8 is insufficient in the current efficiency, and has a problem with the long-term usage because of oxidative deterioration.

A problem of the membrane disclosed in Patent Literature 9 is that the electric resistance is high.

The result shown in an Example of Patent Literature 10 cannot be said to exhibit a sufficiently low internal resistance (electric resistance) of the membrane, and has the problem of the oxidative deterioration resistance in the long-term usage.

Electrolyte membranes (separation membranes) for conventional vanadium-type redox flow batteries are used for the purpose of suppressing the diffusion, migration, and permeation of active substance ions to counter electrodes (cells), and allowing protons (H⁺) to selectively permeate along with the operation of charge and discharge as the purpose, in each of a cell (negative electrode side) in which ions of a low-valent group of vanadium ions, which are active substances of electrolyte solutions of both electrodes, hold a large majority, and a cell (positive electrode side) in which ions of a high-valent group of the vanadium ions hold a large majority. However, the performance cannot be said to be sufficient at present.

As a membrane base material composed mainly of a hydrocarbon-based resin, there are used a porous membrane which only simply separates electrolyte solutions containing electrolytes as principal parts of both cells and exhibits no ion permselectivity, a (nonporous) hydrophilic membrane base material exhibiting no ion permselectivity, a porous membrane having a hydrophilic membrane base material embedded therein or covered thereon, and the like. There are also used as a separation membrane a so-called cation-exchange membrane in which the membrane itself has various types of anion groups, or a composite membrane in which a cation-exchange resin is covered on or embedded in pores of a porous membrane base material, an anion-exchange membrane in which the membrane itself similarly has cation groups, or a composite membrane in which an anion-exchange resin is similarly covered on or embedded in a porous membrane base material, a membrane of a laminate type of both, and the like; and studies making the most of respective features are being carried out.

No ion-exchange resin separation membrane as the separation membrane has been developed so far which sufficiently satisfies two contrary performances of the electric resistance (depending mainly on the proton permeability) and the permeability inhibition of metal ions (polyvalent cations), which are active substances as the principal parts, and further no ion-exchange resin separation membrane has been developed so far which satisfies, in addition to the above two performances, the oxidative deterioration resistance (hydroxy radical resistance) over a long period. Also for fluorine-based ion-exchange resins, no sufficient studies of devices have been made on mutually contradictory properties of the excellent proton (H⁺) permeability and the inhibition of the active substance ion permeation; and no redox flow battery and no electrolyte membrane therefor have been developed which sufficiently satisfy a low electric resistance, a high current efficiency, the oxidative deterioration resistance (hydroxy radical resistance), and the like.

In consideration of the above-mentioned situation, it is an object of the present invention to provide a redox flow secondary battery being low in the electric resistance and excellent in the current efficiency, and further having the durability; and it is an object of the present invention to provide an electrolyte membrane for a redox flow secondary battery having the excellent ion permselectivity capable of suppressing the active substance ion permeability without deteriorating the proton (H⁺) permeability, and further having the oxidative deterioration resistance (hydroxy radical resistance) as well.

Solution to Problem

As a result of exhaustive studies to solve the above-mentioned problems, the present inventors have found that an electrolyte membrane can be provided which contains a fluorine-based polyelectrolyte polymer having a specific structure, has an excellent ion permselectivity by regulating an amount of the fluorine ions eluted in a specific range in an immersion test using a Fenton's reagent solution, and is further excellent also in the oxidative deterioration resistance (hydroxy radical resistance). It has been further found that a redox flow secondary battery can be provided which is low in the electric resistance, excellent in the current efficiency, and further excellent in the durability by using the above electrolyte membrane as the separation membrane, and these findings have led to the completion of the present invention.

That is, the present invention is as follows.

[1]

A redox flow secondary battery comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein in a test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, an amount of fluorine ions eluted detected in a solution is 0.03% or smaller of a whole amount of fluorine in a immersed polymer.

[2]

The redox flow secondary battery according to above [1], wherein in the test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of the Fenton's reagent solution containing the 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, the amount of fluorine ions eluted detected in the solution is 0.002% or smaller of the whole amount of fluorine in the immersed polymer.

[3]

A redox flow secondary battery comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises 0.1 to 20 parts by mass of a polyphenylene ether resin and/or a polyphenylene sulfide resin with respect to 100 parts by mass of the fluorine-based polyelectrolyte polymer.

[4]

A redox flow secondary battery comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Ce-based additive.

[5]

A redox flow secondary battery comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Co-based and/or a Mn-based additive.

[6]

The redox flow secondary battery according to any of above [1] to [5], wherein sulfuric acid electrolyte solutions comprising vanadium are used as the positive electrode electrolyte solution and the negative electrode electrolyte solution.

[7]

The redox flow secondary battery according to any of above [1] to [6], wherein the fluorine-based polyelectrolyte polymer is a perfluorocarbonsulfonic acid resin (PFSA) having a structure represented by the following formula (2): —[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1; and m represents an integer of 1 to 6. [8]

The redox flow secondary battery according to any of above [1] to [7], wherein the fluorine-based polyelectrolyte polymer has an equivalent weight EW (dry mass in grams per equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and the electrolyte membrane has an equilibrium moisture content of 5 to 80% by mass.

[9]

An electrolyte membrane for a redox flow secondary battery, comprising an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein in a test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, an amount of fluorine ions eluted detected in a solution is 0.03% or smaller of a whole amount of fluorine in a immersed polymer.

[10]

The electrolyte membrane for a redox flow secondary battery according to above [9], wherein in the test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of the Fenton's reagent solution containing the 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, the amount of fluorine ions eluted detected in the solution is 0.002% or smaller of the whole amount of fluorine in the immersed polymer.

[11]

An electrolyte membrane for a redox flow secondary battery, comprising an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises 0.1 to 20 parts by mass of a polyphenylene ether resin and/or a polyphenylene sulfide resin with respect to 100 parts by mass of the fluorine-based polyelectrolyte polymer.

[12]

An electrolyte membrane for a redox flow secondary battery, comprising an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Ce-based additive.

[13]

An electrolyte membrane for a redox flow secondary battery, comprising an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Co-based and/or a Mn-based additive.

[14]

The electrolyte membrane for a redox flow secondary battery according to any of above [9] to [13], wherein the fluorine-based polyelectrolyte polymer is a perfluorocarbonsulfonic acid resin (PFSA) having a structure represented by the following formula (2): —[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1; and m represents an integer of 1 to 6. [15]

The electrolyte membrane for a redox flow secondary battery according to any of above [9] to [14], wherein the fluorine-based polyelectrolyte polymer has an equivalent weight EW (dry mass in grams per equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and the electrolyte membrane has an equilibrium moisture content of 5 to 80% by mass.

[16]

The electrolyte membrane for a redox flow secondary battery according to any of above [9] to [15], wherein the electrolyte membrane is subjected to a heat treatment at 130 to 200° C. for 1 to 60 min.

Advantageous Effects of Invention

The redox flow secondary battery according to the present invention is low in the electric resistance and high in the current efficiency, and can further suppress the elimination of ion groups, the collapse phenomenon of a polyelectrolyte, and the like as compared with redox flow secondary batteries using a hydrocarbon-based electrolyte as a separation membrane, and is excellent in the durability.

Since the electrolyte membrane for a redox flow secondary battery according to the present invention has excellent ion permselectivity, is excellent in the high proton (H⁺) permeability and the permeation inhibition of active substance ions in electrolyte solutions, and is further excellent in the oxidative deterioration resistance (hydroxy radical resistance) over a long period, the use of the electrolyte membrane as a separation membrane of a redox flow secondary battery can provide the redox flow secondary battery low in the cell electric resistance and high in the current efficiency; and since the electrolyte membrane exhibits a high oxidative deterioration prevention effect to hydroxy radicals generated in electrolyte solution cells in a system over a long period, the electrolyte membrane can suppress the elimination of ion groups, the collapse phenomenon of the polyelectrolyte, and the like, which are caused in the case of using usual hydrocarbon-based electrolytes.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 shows an example of a schematic diagram of a redox flow secondary battery in the present embodiments.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments to carry out the present invention (hereinafter, referred to as “present embodiments”) will be described in detail. The present invention is not limited to the following present embodiments. Hereinafter, the present embodiments 1 to 4 will be described in order.

The Present Embodiment 1 Redox Flow Secondary Battery

A redox flow secondary battery in the present embodiment 1, comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein in a test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, an amount of fluorine ions eluted detected in the solution is 0.03% or smaller of the whole amount of fluorine in the immersed polymer.

FIG. 1 shows an example of a schematic diagram of a redox flow secondary battery in the present embodiment 1. A redox flow secondary battery 10 in the present embodiment 1 has an electrolytic bath 6 which comprises a positive electrode cell chamber 2 comprising a positive electrode 1 composed of a carbon electrode, a negative electrode cell chamber 4 comprising a negative electrode 3 composed of a carbon electrode, and an electrolyte membrane 5 as a separation membrane to separate the positive electrode cell chamber 2 and the negative electrode cell chamber 4, wherein the positive electrode cell chamber 2 contains a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber 4 contains a negative electrode electrolyte solution comprising a negative electrode active substance. The positive electrode electrolyte solution and the negative electrode electrolyte solution comprising the active substances are, for example, stored in a positive electrode electrolyte solution tank 7 and a negative electrode electrolyte solution tank 8, and fed to respective cell chambers (arrows A and B) by pumps or the like. The current generated by the redox flow secondary battery may be converted from direct current to alternating current through an AC/DC converter 9.

The redox flow secondary battery in the present embodiment 1 has a structure in which each of liquid-permeable porous current collector electrodes (for the negative electrode and for the positive electrode) is disposed on either side of the separation membrane, and these are held by pressing; one side partitioned by the separation membrane is made the positive electrode cell chamber and the other side is made the negative electrode cell chamber; and the thicknesses of both the cell chambers are secured by spacers.

In the case of a vanadium-type redox flow secondary battery, the charge and discharge of the battery is carried out by circulating the positive electrode electrolyte solution composed of a sulfuric acid electrolyte solution comprising tetravalent vanadium (e) and pentavalent vanadium (V⁵⁺) to the positive electrode cell chamber, and circulating the negative electrode electrolyte solution comprising trivalent vanadium (V³⁺) and divalent vanadium (V²⁺) to the negative electrode cell chamber. In the charge time therein, in the positive electrode cell chamber, vanadium ions release electrons to thereby oxidize V⁴⁺ to V⁵⁺; and in the negative electrode cell chamber, electrons having returned through an external circuit reduce V³⁺ to V²⁺. In the oxidation and reduction reactions, in the positive electrode cell chamber, protons (W) become excessive; by contrast, in the negative electrode cell chamber, protons (H⁺) become insufficient. The excessive protons in the positive electrode cell chamber selectively migrate to the negative electrode chamber through the separation membrane to thereby hold the electric neutrality. In the discharge time, a reaction reverse thereto progresses. The battery efficiency (%) at this time is represented by a ratio (%) obtained by dividing a discharge electric energy by a charge electric energy; and both the electric energies depend on the internal resistance of the battery cells, the ion permselectivity of the separation membrane, and the current losses of others. Since the reduction of the internal resistance improves the voltage efficiency, and the improvement of the ion permselectivity and the reduction of the current losses of others improve the current efficiency, these factors become important indices in the redox flow secondary battery.

[Electrolyte Membrane for a Redox Flow Secondary Battery]

The electrolyte membrane for a redox flow secondary battery in the present embodiment 1 has a specific structure, and comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer a part of whose molecular chain terminals is fluorinated.

<Ion-Exchange Resin Composition>

In the present embodiment 1, the ion-exchange resin composition comprises a fluorine-based polyelectrolyte polymer having a structure represented by the above formula (1).

(Fluorine-Based Polyelectrolyte Polymer)

In the present embodiment 1, the fluorine-based polyelectrolyte polymer has a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time).

X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms. Here, the halogen atoms include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. X¹, X² and X³, from the viewpoint of the chemical stability including the oxidative deterioration resistance of the polymer, are preferably each a fluorine atom or a perfluoroalkyl group having 1 to 3 carbon atoms.

X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ. Hereinafter, X⁴ is also referred to as an “ion-exchange group.” Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄). Here, the alkali metal atom is not especially limited, and includes a lithium atom, a sodium atom, and a potassium atom. The alkaline earth metal atom is not especially limited, and includes a calcium atom and a magnesium atom. R₁, R₇, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups. Here, in the case where X⁴ is PO₃Z₂, Z may be identical or different. X⁴, from the viewpoint of the chemical stability including the oxidative deterioration resistance of the polymer, is preferably SO₃Z.

R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms. Here, the halogen atoms include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.

a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1. b represents an integer of 0 to 8. c represents 0 or 1. d, e, and f each independently represent an integer of 0 to 6. Here, d, e, and f are not 0 at the same time.

The fluorine-based polyelectrolyte polymer in the present embodiment 1 is preferably a perfluorocarbonsulfonic acid resin (hereinafter, also referred to as “PFSA resin”) because of giving a tendency of making the advantage of the present invention more remarkable. The PFSA resin in the present embodiment is a resin in which perfluorocarbons as side chains are bonded to the main chain composed of a PTFE skeleton chain, and one or two or more sulfonic acid groups (as the case may be, a part of the groups may be a form of a salt) are bonded to the each side chain.

The PFSA resin preferably comprises a repeating unit represented by —(CF₂—CF₂)— and a repeating unit derived from a compound represented by the following formula (3) or (4), and is further preferably composed of a repeating unit represented by —(CF₂—CF₂)— and a repeating unit derived from a compound represented by the formula (3) or (4).

Formula (3): CF₂═CF(—O—(CF₂CFXO)_(n)— [A]) wherein X represents F or a perfluoroalkyl group having 1 to 3 carbon atoms; n represents an integer of 0 to 5; and [A] is (CF₂)_(m)—SO₃H wherein m represents an integer of 1 to 6, here, n and m are not 0 at the same time, or Formula (4): CF₂═CF—O—(CF₂)_(P)—CFX(—O—(CF₂)_(K)SO₃H) or CF₂═CF—O—(CF₂)_(P)—CFX(—(CF₂)_(L)—O—(CF₂)_(m)—SO₃H) wherein X represents a perfluoroalkyl group having 1 to 3 carbon atoms; and P represents an integer of 0 to 12, K represents an integer of 1 to 5, L represents an integer of 1 to 5, and m represents an integer of 0 to 6, here, K and L may be identical or different, and P, K, and L are not 0 at the same time.

The PFSA resin is a copolymer comprising a repeating unit represented by —(CF₂—CF₂)— and a repeating unit represented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— wherein X represents F or CF₃; and n represents an integer of 0 to 5, and m represents an integer of 0 to 12, here, n and m are not 0 at the same time, and is more preferably a copolymer necessarily comprising a repeating unit represented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— wherein X represents CF₃; and n represents 0 or 1, and m represents an integer of 0 to 12, here, n and m are not 0 at the same time. The case where the PFSA resin is a copolymer having the above structure and has a predetermined equivalent weight EW has such tendencies that an obtained electrolyte membrane exhibits sufficient hydrophilicity, and the resistance to radical species generated by oxidative deterioration becomes high.

The case where the PFSA resin comprises the repeating unit of —(CF₂—CF(—O—(CF₂CFXO)_(n)— (CF₂)_(m)—SO₃H))— wherein n is 0 and m is an integer of 1 to 6, or both the repeating units of —CF₂—CF(—O—(CF₂)_(P)—CFX(—O—(CF₂)_(K)—SO₃H)— and —CF₂—CF(—O—(CF₂)_(P)—CFX(—(CF₂)_(L)—O—(CF₂)_(m)—SO₃H)— represented by the formula (4) has further such tendencies that the equivalent weight EW becomes low and the hydrophilicity of an obtained electrolyte membrane becomes high.

In the copolymer, of Nafion (registered trademark of Du Pont K.K.) which is a fluorine-based resin used in the conventional technology, containing a repeating unit represented by —(CF₂—CF₂)— and a repeating unit of —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))—, it is known that X═CF₃, n=1 and m=2; and the EW described later is 893 to 1,030.

It has been found as a result of studies by the present inventors that in the case where a PFSA resin is used as an electrolyte membrane for a redox flow secondary battery, the PFSA resin comprising the repeating unit represented by —(CF₂—CF(—O—(CF₂CFXO)_(n)—(CF₂)_(m)—SO₃H))— wherein n is 0 and m is an integer of 1 to 6, or both the repeating units of —CF₂—CF(—O—(CF₂)_(P)—CFX(—O—(CF₂)_(K)—SO₃H)— and —CF₂—CF(—O—(CF₂)₂—CFX(—(CF₂)_(L)—O—(CF₂)_(m)—SO₃H)— represented by the formula (4) has such tendencies that the hydrophilicity and the ion permselectivity are excellent, and the electric resistance of an obtained redox flow secondary battery is low and the current efficiency thereof is improved, as compared with the above Nafion.

The fluorine-based polyelectrolyte polymer represented by the formula (1) in the present embodiment 1 is preferably a PFSA resin having a structure represented by the following formula (2) because of giving a tendency of making the advantage of the present invention more remarkable. —[CF₂CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1; m represents an integer of 1 to 6.

The fluorine-based polyelectrolyte polymer represented by the above formula (1) and the PFSA resin having a structure represented by the above formula (2) in the present embodiment 1, respectively, are not especially limited as long as having the structures represented by the above formula (1) and the above formula (2), and may comprise other structures.

The fluorine-based polyelectrolyte polymer represented by the above formula (1) and the PFSA resin having a structure represented by the above formula (2) in the present embodiment 1 may be those in which a part of ion-exchange groups is subjected to intermolecular direct or indirect partial crosslinking reaction. The partial crosslinking is preferable from the viewpoint of being able to control the solubility and the excessive swell.

For example, even if the EW of a fluorine-based polyelectrolyte polymer is about 280, by carrying out the above partial crosslinking, the solubility of the fluorine-based polyelectrolyte polymer to water can be reduced (the water resistance can be improved).

Also in the case where a fluorine-based polyelectrolyte polymer is in a low melt flow region (polymer region), the above partial crosslinking can increase intermolecular entanglement and reduce the solubility and the excessive swell.

Examples of the partial crosslinking reaction include a reaction of an ion-exchange group with a functional group or the main chain of another molecule, a reaction of ion-exchange groups, and a crosslinking reaction (covalent bond) through an oxidation-resistant low molecular compound, oligomer, polymeric substance, or the like, and as the case may be, a reaction with a substance to form a salt (including an ionic bond with a SO₃H group). Examples of the oxidation-resistant low molecular compound, oligomer, and polymeric substance include polyhydric alcohols and organic diamines.

(Equivalent Weight EW of a Fluorine-Based Polyelectrolyte Polymer)

The equivalent weight EW (dry mass in grams of a fluorine-based polyelectrolyte polymer per equivalent weight of an ion-exchange group) of the fluorine-based polyelectrolyte polymer in the present embodiment 1 is preferably 300 to 1,300 (g/eq), more preferably 350 to 1,000 (g/eq), still more preferably 400 to 900 (g/eq), and especially preferably 450 to 750 (g/eq).

In a fluorine-based polyelectrolyte polymer having a structure of the above formula (1), by regulating the equivalent weight EW thereof in the above range, an ion-exchange resin composition containing the polymer can be imparted with excellent hydrophilicity; and an electrolyte membrane obtained by using the resin composition results in having a lower electric resistance and a higher hydrophilicity, and having a large number of smaller clusters (minute moieties where ion-exchange groups coordinate and/or adsorb water molecules), and gives such a tendency that the oxidation resistance (hydroxy radical resistance) and the ion permselectivity are more improved.

The equivalent weight EW of a fluorine-based polyelectrolyte polymer is preferably 300 or higher from the viewpoint of the hydrophilicity and the water resistance of the membrane; and that is preferably 1,300 or lower from the viewpoint of the hydrophilicity and the electric resistance of the membrane.

The equivalent weight EW of a fluorine-based polyelectrolyte polymer can be measured by replacing the fluorine-based polyelectrolyte polymer by a salt, and back-titrating the solution with an alkali solution.

The equivalent weight EW can be regulated by selecting copolymerization ratios of fluorine-based monomers as raw materials of a fluorine-based polyelectrolyte polymer, kinds of the monomers, and the like.

(Method for Producing a Fluorine-Based Polyelectrolyte Polymer)

A fluorine-based polyelectrolyte polymer in the present embodiment 1 can be obtained, for example, by producing a precursor of a polyelectrolyte polymer (hereinafter, also referred to as “resin precursor”), and thereafter subjecting the precursor to a hydrolysis treatment.

A PFSA resin can be obtained, for example, by hydrolyzing a PFSA resin precursor composed of a copolymer of a fluorinated vinyl ether compound represented by the following general formula (5) or (6) with a fluorinated olefin monomer represented by the following general formula (7). CF₂═CF—O—(CF₂CFXO)_(n)-A  Formula (5): wherein X represents F or a perfluoroalkyl group having 1 to 3 carbon atoms; n represents an integer of 0 to 5; and A represents (CF₂)_(m)—W, and W represents a functional group capable of being converted to SO₃H by hydrolysis. CF₂═CF—O—(CF₂)_(P)—CF((—O—(CF₂)_(K)—W) or CF₂═CF—O—(CF₂)_(P)—CF(—(CF₂)_(L)—O—(CF₂)_(m)—W)  Formula (6): wherein p represents an integer of 0 to 12, and m represents an integer of 0 to 6, here, n and m are not 0 at the same time; K represents an integer of 1 to 5; L represents an integer of 1 to 5, here, n and L or K are not 0 at the same time; and W represents a functional group capable of being converted to SO₃H by hydrolysis. CF₂═CFZ  Formula (7): wherein Z represents H, Cl, F, a perfluoroalkyl group having 1 to 3 carbon atoms, or a cyclic perfluoroalkyl group which may contain oxygen.

W denoting a functional group capable of being converted to SO₃H by hydrolysis in the above formula (5) is not especially limited, but is preferably SO₂F, SO₂Cl, or SO₂Br. Further in the above formulae, X═CF₃, W=SO₂F, and Z═F are more preferable. Particularly, n=0, m=an integer of 1 to 6, X═CF₃, W=SO₂F, and Z═F are especially preferable because of giving tendencies of providing high hydrophilicity and a solution having a high resin concentration.

The above resin precursor in the present embodiment 1 can be synthesized by well-known means. The resin precursor can be produced, for example, by polymerizing a fluorinated vinyl compound having a group (ion-exchange group precursor group) capable of being converted to an ion-exchange group (X⁴ in the formula (1)) by hydrolysis or the like in the presence of a radical generator such as a peroxide or the like, with a fluorinated olefin such as tetrafluoroethylene (TFE). The polymerization method is not especially limited, and usable methods thereof include a method (solution polymerization) of filling and dissolving and reacting the fluorinated vinyl compound or the like and a gas of the fluorinated olefin in a polymerization solvent such as a fluorine-containing hydrocarbon, to thereby carry out the polymerization, a method (bulk polymerization) of carrying out the polymerization by using the fluorinated vinyl compound itself as a polymerization solvent without using any solvent such as a fluorine-containing hydrocarbon, a method (emulsion polymerization) of filling and reacting the fluorinated vinyl compound and a gas of the fluorinated olefin by using an aqueous solution of a surfactant as a medium, to thereby carry out the polymerization, a method (emulsion polymerization) of filling and emulsifying and reacting the fluorinated vinyl compound and a gas of the fluorinated olefin in an aqueous solution of a surfactant and an emulsifying aid such as an alcohol to thereby carry out the polymerization, and a method (suspension polymerization) of filling and suspending and reacting the fluorinated vinyl compound and a gas of the fluorinated olefin in an aqueous solution of a suspension stabilizer to thereby carry out the polymerization.

In the present embodiment 1, any resin precursor fabricated by any polymerization method described above can be used. Any block-shape or taper-shape polymer obtained by regulating the polymerization condition such as the amount of TFE gas supplied may be used as the resin precursor.

The resin precursor may be one prepared by treating impure terminals and structurally easily-oxidizable moieties (CO group-, H-bonded moieties and the like) produced in a resin molecular structure during the polymerization reaction by a well-known method under fluorine gas to thereby fluorinate the moieties.

In the resin precursor, a part of ion-exchange group precursor groups (for example, SO₂F groups) may be partially (including intermolecularly) imidized (e.g., alkylimidized).

The molecular weight of the resin precursor is not especially limited, but in terms of a value of a melt flow index (MFI) of the precursor measured according to ASTM: D1238 (measurement conditions: a temperature of 270° C. and a load of 2,160 g), is preferably 0.05 to 50 (g/10 min), more preferably 0.1 to 30 (g/10 min), and still more preferably 0.5 to 20 (g/10 min).

The shape of the resin precursor is not especially limited, but from the viewpoint of accelerating treatment rates in a hydrolysis treatment and an acid treatment described later, is preferably a pellet-shape of 0.5 cm³ or smaller, a disperse liquid or a powdery particle-shape; and among these, powdery bodies after the polymerization are preferably used. From the viewpoint of the costs, an extruded film-shape resin precursor may be used.

A method for producing a fluorine-based polyelectrolyte polymer of the present embodiment 1 from the resin precursor is not especially limited, and examples thereof include a method in which the resin precursor is extruded through a nozzle, a die, or the like by using an extruder, and thereafter is subjected to a hydrolysis treatment, and a method in which the resin precursor product as it is on the polymerization, that is, a disperse-liquid product, or a product made powdery by precipitation and filtration is thereafter subjected to a hydrolysis treatment.

More specifically, a resin precursor obtained as in the above, and as required, molded is then immersed in a basic reaction liquid to be thereby subjected to a hydrolysis treatment. The basic reaction liquid used in the hydrolysis treatment is not especially limited, but preferable are an aqueous solution of an amine compound such as dimethylamine, diethyleamine, monomethylamine, or monoethylamine, and an aqueous solution of a hydroxide of an alkali metal or an alkaline earth metal; and especially preferable are aqueous solutions of sodium hydroxide and potassium hydroxide. In the case of using a hydroxide of an alkali metal or an alkaline earth metal, the content thereof is not especially limited, but preferably 10 to 30% by mass with respect to the whole of a reaction liquid. The reaction liquid more preferably further contains a swelling organic compound such as methyl alcohol, ethyl alcohol, acetone, and dimethyl sulfoxide (DMSO). The content of a swelling organic compound is preferably 1 to 30% by mass with respect to the whole of the reaction liquid.

The resin precursor is subjected to a hydrolysis treatment in the basic reaction liquid, thereafter sufficiently washed with warm water or the like, and thereafter subjected to an acid treatment. An acid used in the acid treatment is not especially limited, but is preferably a mineral acid such as hydrochloric acid, sulfuric acid, or nitric acid, or an organic acid such as oxalic acid, acetic acid, formic acid, or trifluoroacetic acid, and more preferably a mixture of these acids and water. The above acids may be used singly or in combinations of two or more. A basic reaction liquid used in the hydrolysis treatment may be removed by a treatment with a cation-exchange resin or the like previously before the acid treatment.

An ion-exchange group precursor group of a resin precursor is protonated by an acid treatment to thereby produce an ion-exchange group. For example, in the case of a PFSA resin precursor produced using a fluorinated vinyl ether compound represented by the above formula (5), W in the formula (5) is protonated by an acid treatment to thereby make SO₃H. A fluorine-based polyelectrolyte polymer obtained by the hydrolysis treatment and acid treatment is enabled to be dispersed or dissolved in a protonic organic solvent, water, or a mixed solvent of the both.

In the fluorine-based polyelectrolyte polymer in the present embodiment 1, a part of the molecular chain terminals is fluorinated. Since the decomposition of the fluorine-based polyelectrolyte polymer is suppressed during the operation of a redox flow secondary battery by fluorinating and thereby stabilizing unstable functional groups present at a part of molecular chain terminals of the polymer, an electrolyte membrane for a redox flow secondary battery excellent in the durability can be provided.

A method of fluorinating molecular chain terminals of a fluorine-based polyelectrolyte polymer is not especially limited, but for example, may involve treating impure terminals and structurally easily-oxidizable moieties (CO group-, H-bonded moieties and the like) produced in a resin molecular structure during the above-mentioned polymerization reaction of the fluorine-based polyelectrolyte polymer by a well-known method under fluorine gas to thereby fluorinate the moieties. Specifically, a part of molecular chain terminals can be fluorinated, for example, by subjecting a perfluorocarbonsulfonic acid resin having a precursor group of a sulfonic acid group to a heat treatment at a temperature of 200 to 300° C. under reduced pressure of a pressure of 0.02 MPa or lower for 0.1 hours or longer, and thereafter bringing the heat-treated resin into contact with fluorine gas at a temperature of 150 to 200° C.

Here, the fluorination of molecular chain terminals of a fluorine-based polyelectrolyte polymer can be evaluated by the following Fenton's reagent immersion test.

In a test in which 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions is prepared; thereafter, within 1 min, 0.1 g of the polymer is immersed in the Fenton's reagent solution at 40° C. for 16 hours under no stirring, and the fluorination is evaluated by a variation in the amount of the fluorine ions eluted detected in the solution before and after the test. Here, the amount of the fluorine ions eluted detected in the solution is 0.03% or smaller of the whole amount of fluorine in the polymer immersed, preferably 0.01% or smaller, and more preferably 0.002% or smaller. If the amount of the fluorine ions eluted is 0.03% or smaller of the whole amount of fluorine in the polymer immersed, the amount of unstable terminal groups is small, and the decrease in the voltage hardly occurs, in the battery operation for a long time.

The shape of a sample for the Fenton's reagent immersion test may be a polyelectrolyte polymer, or may be a film prepared by forming as a film a dispersion liquid containing the polymer by a well-known method.

The amount of the fluorine ions eluted makes an index of the resistance to the decomposition of the polymer in the operation of a redox flow secondary battery.

(Ion-Exchange Resin Composition)

The content of a fluorine-based polyelectrolyte polymer having a structure represented by the above formula (1) contained in an ion-exchange resin composition forming an electrolyte membrane in the present embodiment 1 is not especially limited, but the ion-exchange resin composition preferably contains as a main component the fluorine-based polyelectrolyte polymer having the above specific structure from the viewpoint of the ion permselectivity and the oxidative deterioration resistance. Here, “containing as a main component” refers to a lower limit value of the content in the resin composition of about 33.3% by mass, preferably 40% by mass, more preferably 50% by mass, still more preferably 50% by mass, further still more preferably 80% by mass, and especially preferably 90% by mass. The upper limit value is not especially limited, but is preferably 99.5% by mass or less.

An ion-exchange resin composition in the present embodiment 1 may contain fluorine-based resins (fluorine-based resins containing carboxylic acid, phosphoric acid or the like, and other well-known fluorine-based resins) other than a fluorine-based polyelectrolyte polymer represented by the formula (1). The fluorine-based resin is contained, with respect to 100 parts by mass of a fluorine-based polyelectrolyte polymer represented by the formula (1) used in the present embodiment, preferably in 30 to 50 parts by mass, more preferably in 10 to 30 parts by mass, and still more preferably 0 to 10 parts by mass.

In the case of using two or more of these resins, a mixing method is not especially limited, and may involve dissolving in a solvent or dispersing in a medium and mixing the resins, or may involve extrusion-mixing resin precursors.

The fluorine-based polyelectrolyte polymer may be contained singly in a form of a partial salt (about 0.01 to 5 equivalent % of the equivalent of the whole ion-exchange group) with an alkali metal, an alkaline earth metal, or besides, a radical-decomposable transition metal (Ce-based additive, Co-based additive, Mn-based additive, or the like), or in a form in concurrent use therewith of a basic polymer described later.

If an ion-exchange resin composition in the present embodiment 1 contains, in addition to the above-mentioned fluorine-based polyelectrolyte polymer, a basic polymer (including a low molecular weight substance such as an oligomer), the chemical stability (mainly the oxidation resistance and the like) as the resin composition is likely to increase, which is therefore preferable. These compounds partially make ion complexes in a microparticulate form or a form near molecular dispersion in the resin composition, and form an ionically crosslinked structure. Particularly in the case where EW of a fluorine-based polyelectrolyte polymer is low (for example, in the case of 300 to 500), these compounds are preferable from the viewpoint of the balance among the water resistance, the electric resistance, and the like.

(Equilibrium Moisture Content)

The equilibrium moisture content of an electrolyte membrane in the present embodiment 1 is preferably 5% by mass or higher, more preferably 10% by mass or higher, and still more preferably 15% by mass or higher. The equilibrium moisture content of an electrolyte membrane in the present embodiment 1 is preferably 80% by mass or lower, more preferably 50% by mass or lower, and still more preferably 40% by mass or lower. If the equilibrium moisture content of an electrolyte membrane is 5% by mass or higher, the electric resistance, the current efficiency, the oxidation resistance, and the ion permselectivity of the membrane are likely to be good. By contrast, if the equilibrium moisture content is 80% by mass or lower, the dimensional stability and the strength of the membrane are likely to be good and the increase of water-soluble components is likely to be suppressed. The equilibrium moisture content of an electrolyte membrane is expressed as an equilibrium (being left for 24 hours) saturated water absorption rate (Wc) at 23° C. and 50% relative humidity (RH), based on the membrane prepared by forming a membrane from a dispersion liquid of the resin composition with water and an alcoholic solvent, and drying the membrane at 160° C. or lower.

The equilibrium moisture content of an electrolyte membrane can be regulated by the similar method as in EW described above.

(Method for Producing an Electrolyte Membrane)

A production method of an electrolyte membrane (membrane formation method) in the present embodiment 1 is not especially limited, and a well-known extrusion method or cast membrane formation method can be used. The electrolyte membrane may be of a single layer or of a multilayer (2 to 5 layers); and in the case of a multilayer, the performance of the electrolyte membrane can be improved by laminating membranes having different properties (for example, resins having different EWs and functional groups). In the case of a multilayer, the lamination may be carried out at the extrusion membrane production time or the cast time, or each membrane obtained may be laminated.

The electrolyte membrane formed in the above method is sufficiently washed with water (or, as required, before water washing, treated with an aqueous acidic liquid such as dilute hydrochloric acid, nitric acid, or sulfuric acid) to thereby remove impurities, and is preferably subjected to a heat treatment in the air or an inert gas (preferably in an inert gas) preferably at 130 to 200° C., more preferably at 140 to 180° C., and still more preferably 150 to 170° C., for 1 to 60 min. The time of the heat treatment is more preferably 1 to 30 min, still more preferably 2 to 20 min, further still more preferably 3 to 15 min, and especially preferably about 5 to 10 min.

The resin in the state as it is at the membrane formation time is usually not sufficiently entangled among particles (among primary particles and secondary particles) and molecules originated from raw materials. One reason for carrying out the above heat treatment is the purpose of interparticulately and intermolecularly entangling the resin, and is particularly in stabilizing the water resistance (particularly decreasing the hot water-dissolving component ratio) and the saturated water absorption rate of water, and producing stable clusters. The heat treatment is useful also from the viewpoint of the improvement of the membrane strength. Particularly in the case of using the cast membrane formation method, the heat treatment is useful.

Another reason for carrying out the above heat treatment is because the formation of fine intermolecular crosslinking among molecules of a fluorine-based polyelectrolyte polymer presumably contributes to the formation of clusters excellent in the water resistance and stable, and provides an effect of making the cluster diameter uniform and small.

A further reason is because the above heat treatment presumably causes at least a part of ion-exchange groups of a fluorine-based polyelectrolyte polymer in an ion-exchange resin composition to react with active reaction sites (aromatic rings and the like) of other additive (including resins) components to thereby form fine crosslinking through the reaction (particularly the reaction of ion-exchange groups present near the other resin components being dispersed additives) and contribute to the stabilization. The degree of the crosslinking is, in terms of EW (the degree of the EW decrease before and after the heat treatment), preferably 0.001 to 5%, more preferably 0.1 to 3%, and still more preferably about 0.2 to 2%.

Carrying out the above heat treatment under the above suitable condition (time, temperature) is preferable from the viewpoint of exhibiting the effect of the heat treatment, and from the viewpoint of suppressing the degradation of the oxidative deterioration resistance during actual usage as an electrolyte membrane, with the degradation starting from faults generated in the molecular structure due to the generation and increase of fluorine removal, hydrofluoric acid removal, sulfonic acid removal and thermally oxidized sites, and the like.

The Present Embodiment 2

A redox flow secondary battery in the present embodiment 2, comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Ce-based additive.

An ion-exchange resin composition forming an electrolyte membrane in the present embodiment 2 contains a Ce-based additive. The incorporation of the Ce-based additive in the ion-exchange resin composition conceivably ion-exchanges cerium ions for a part of ion-exchange groups contained in the electrolyte membrane, resulting in improving the ion permselectivity and the oxidative deterioration resistance.

The Ce-based additive is preferably one to form cerium ions having a valence of +3 and/or +4 in a solution. Examples of salts containing cerium ions having a valence of +3 include cerium nitrate, cerium carbonate, cerium acetate, cerium chloride, and cerium sulfate. Examples of salts containing cerium ions having a valence of +4 include cerium sulfate (Ce(SO₄)₂.4H₂O), cerium diammonium nitrate (Ce(NH₄)₂(NO₃)₆), and cerium tetraammonium sulfate (Ce(NH₄)₄(SO₄)₄.4H₂O). As a Ce-based additive, an organometal complex salt of cerium can also be used, and an example thereof includes cerium acetylacetonate (Ce(CH₃COCHCOCH₃)₃.3H₂O). Among the above, cerium nitrate and cerium sulfate are especially preferable because of being water-soluble and being likely to be easily handled.

The content of a Ce-based additive is, in a proportion of cerium ions with respect to the number of ion-exchange groups in an electrolyte membrane, preferably 0.02 to 20%, more preferably 0.05 to 15%, and still more preferably 0.07 to 10%. If the content of a Ce-based additive is 20% or lower, the ion permselectivity is likely to be good; and if 0.02% or higher, the oxidative deterioration resistance (hydroxy radical resistance) is likely to be improved.

If an ion-exchange resin composition in the present embodiment 2 contains, in addition to the above-mentioned fluorine-based polyelectrolyte polymer and the Ce-based additive, an alkali metal, an alkaline earth metal, a radical-decomposable transition metal (Co-based additive, Mn-based additive, or the like), and a basic polymer (including a low molecular weight substance such as an oligomer), the chemical stability (mainly the oxidation resistance and the like) as the resin composition is likely to increase. These compounds partially make ion complexes in a microparticulate form or a form near molecular dispersion in the resin composition, and form an ionically crosslinked structure. Particularly in the case where EW of a fluorine-based polyelectrolyte polymer is low (in the case of 300 to 500), these compounds are preferable from the viewpoint of the balance among the water resistance, the electric resistance, and the like.

In the present embodiment 2, since each member constituting a redox flow secondary battery and an electrolyte membrane, and physical properties thereof are similar to those in the present embodiment 1, the description thereof will be omitted.

The Present Embodiment 3

A redox flow secondary battery in the present embodiment 3, comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises a Co-based and/or a Mn-based additive.

An ion-exchange resin composition forming an electrolyte membrane in the present embodiment 3 contains a Co-based and/or a Mn-based additive. The incorporation of the Co-based and/or the Mn-based additive in the ion-exchange resin composition conceivably ion-exchanges cobalt ions and/or manganese ions for a part of ion-exchange groups contained in the electrolyte membrane, resulting in improving the ion permselectivity and the oxidative deterioration resistance.

The Co-based additive is preferably one to form cobalt ions having a valence of +2 and/or +3 in a solution. Examples of salts containing cobalt ions having a valence of +2 include cobalt nitrate, cobalt carbonate, cobalt acetate, cobalt chloride, and cobalt sulfate. Examples of salts containing cobalt ions having a valence of +3 include cobalt chloride (CoCl₃) and cobalt nitrate (Co(NO₃)₂). As a Co-based additive, an organometal complex salt of cobalt can also be used, and an example thereof includes cobalt acetylacetonate (Co(CH₃COCHCOCH₃)₃). Among the above, cobalt nitrate and cobalt sulfate are preferable because of being water-soluble and being likely to be easily handled.

As the Mn-based additive, various types of compounds can be used such as water-soluble manganese salts, water-insoluble manganese salts, and insoluble compounds including oxides and hydroxides. The valence of manganese is +2 or +3. Examples of salts containing manganese ions having a valence of +2 include manganese acetate (Mn(CH₃COO)₂.4H₂O), manganese chloride (MnCl₂.4H₂O), manganese nitrate (Mn(NO₃)₂.6H₂O), manganese sulfate (MnSO₄.5H₂O), and manganese carbonate (MnCO₃.nH₂O). Examples of salts containing manganese ions having a valence of +3 include manganese acetate (Mn(CH₃COO)₃.2H₂O). Organometal complex salts of manganese can also be used, and an example thereof includes manganese acetylacetonate (Mn(CH₃COCHCOCH₃)₂). Among the above, manganese nitrate and manganese sulfate are preferable because of being water-soluble and being likely to be easily handled.

The content of a Co-based and/or a Mn-based additive is, in a proportion of cobalt ions and/or manganese ions with respect to the number of ion-exchange groups in an electrolyte membrane, preferably 0.01 to 50%, more preferably 0.05 to 30%, and still more preferably 0.07 to 20%. If the content of a Co-based and/or a Mn-based additive is 50% or lower, the ion permselectivity is likely to be good; and if 0.01% or higher, the oxidative deterioration resistance (hydroxy radical resistance) is likely to be improved.

If an ion-exchange resin composition in the present embodiment contains, in addition to the above-mentioned fluorine-based polyelectrolyte polymer and the Co-based and/or Mn-based additive, an alkali metal, an alkaline earth metal, a radical-decomposable transition metal (Ce-based additive and the like), and a basic polymer (including a low molecular weight substance such as an oligomer), the chemical stability (mainly the oxidation resistance and the like) as the resin composition is likely to increase. These compounds partially make ion complexes in a microparticulate form or a form near molecular dispersion in the resin composition, and form an ionically crosslinked structure. Particularly in the case where EW of a fluorine-based polyelectrolyte polymer is low (in the case of 300 to 500), these compounds are preferable from the viewpoint of the balance among the water resistance, the electric resistance, and the like.

In the present embodiment 3, since each member constituting a redox flow secondary battery and an electrolyte membrane, and physical properties thereof are similar to those in the present embodiment 1, the description thereof will be omitted.

The Present Embodiment 4

A redox flow secondary battery in the present embodiment 4, comprising an electrolytic bath comprising:

a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode;

a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and

an electrolyte membrane as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber,

wherein the positive electrode cell chamber comprises a positive electrode electrolyte solution comprising a positive electrode active substance; and the negative electrode cell chamber comprises a negative electrode electrolyte solution comprising a negative electrode active substance;

wherein the redox flow secondary battery charges and discharges based on changes in valences of the positive electrode active substance and the negative electrode active substance in the electrolyte solutions;

wherein the electrolyte membrane comprises an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents COOZ, SO₃Z, PO₃Z₂, or PO₃HZ wherein Z represents a hydrogen atom, an alkali metal atom, an alkaline earth metal atom, or amines (NH₄, NH₃R₁, NH₂R₁R₂, NHR₁R₂R₃, NR₁R₂R₃R₄) wherein R₁, R₂, R₃, and R₄ each independently represent one or more selected from the group consisting of alkyl groups and arene groups, when X⁴ is PO₃Z₂, Z may be identical or different; R¹ and R² each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and

wherein the ion-exchange resin composition comprises 0.1 to 20 parts by mass of a polyphenylene ether resin and/or a polyphenylene sulfide resin with respect to 100 parts by mass of the fluorine-based polyelectrolyte polymer.

An ion-exchange resin composition in the present embodiment 4, from the viewpoint of the oxidation resistance and the cluster diameter of an electrolyte membrane, contains a polyphenylene ether resin (hereinafter, also referred to as “PPE resin”) and/or a polyphenylene sulfide resin (hereinafter, also referred to as “PPS resin”).

PPE and PPS resins can be added by a method of mixing these to a resin composition containing a fluorine-based polyelectrolyte polymer by an extrusion method, or a method of mixing an aqueous solvent dispersion of PPE and PPS resins to a stock dispersion of a resin composition containing a fluorine-based polyelectrolyte polymer.

The amount of PPE and PPS resins added, with respect to 100 parts by mass of a fluorine-based polyelectrolyte polymer having a structure represented by the above formula (1), is 0.1 to 20 parts by mass, and preferably 0.5 to 10 parts by mass. In the case where the content of a PPE resin and/or a PPS resin is 0.1 parts by mass or higher, the oxidation resistance and the ion permselectivity of an electrolyte membrane are improved; and in the case of 20 parts by mass or lower, a sufficient membrane strength can be provided.

The PPS resin in the present embodiment is preferably a PPS resin containing 70 mol % or more of a paraphenylene sulfide skeleton, and preferably 90 mol % or more thereof. A method for producing a PPS resin is not especially limited, and includes a method in which usually a halogen-substituted aromatic compound, for example, p-dichlorobenzene is polymerized in the presence of sulfur and sodium carbonate, a method of the polymerization in the presence of sodium sulfide or sodium hydrosulfide and sodium hydroxide in a polar solvent, a method of the polymerization in the presence of hydrogen sulfide and sodium hydroxide or sodium aminoalkanoate in a polar solvent, and a method of self-condensation of p-chlorothiophenol; but among these, a method is suitable in which sodium sulfide and p-dichlorobenzene are reacted in an amide-based solvent such as N-methylpyrrolidone or dimethylacetamide, or a sulfone-based solvent such as sulfolane. Specifically, there can be used methods described, for example, in U.S. Pat. No. 2,513,188, Japanese Patent Publication Nos. 44-27671, 45-3368, and 52-12240, Japanese Patent Laid-Open No. 61-225217, U.S. Pat. No. 3,274,165, British Patent No. 1160660, Japanese Patent Publication No. 46-27255, Belgian Patent No. 29437, and Japanese Patent Laid-Open No. 5-222196, and there can be used methods of prior arts exemplified in these official gazettes.

The melt viscosity (a value acquired by holding at 300° C. and a load of 196 N for 6 min using a flow tester whose L/D (L: orifice length, D: orifice diameter)=10/1) at 320° C. of a PPS resin is preferably 1 to 10,000 poises, and more preferably 100 to 10,000 poises.

A PPS resin into which an acidic functional group has been incorporated can further be used suitably. As an acidic functional group to be incorporated, preferable are a sulfonic acid group, a phosphoric acid group, a carboxylic acid group, a maleic acid group, a maleic anhydride group, a fumaric acid group, an itaconic acid group, an acrylic acid group, and a methacrylic acid group; and especially preferable is a sulfonic acid group.

A method of incorporating an acidic functional group is not especially limited, and usual methods can be used. The incorporation of a sulfonic group can be carried out, for example, by using a sulfonating agent such as sulfuric anhydride or fuming sulfuric acid under the well-known condition; specifically, the incorporation can be carried out under the conditions described in K. Hu, T. Xu, W. Yang, Y. Fu, Journal of Applied Polymer Science, Vol. 91, and E. Montoneri, Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 27, 3043-3051 (1989).

Also a PPS resin can suitably be used which is prepared by substituting the acidic functional group incorporated in a PPS resin with a metal salt or an amine salt. In this case, as the metal salt, preferably used are alkali metal salts such as sodium salts and potassium salts, and alkaline earth metal salts such as calcium salts.

The PPE resin is not especially limited, and examples thereof include poly(2,6-dimethyl-1,4-phenylene ether), poly(2-methyl-6-ethyl-1,4-phenylene ether), poly(2-methyl-6-phenyl-1,4-phenylene ether), and poly(2,6-dichloro-1,4-phenylene ether), and also include polyphenylene ether copolymers such as copolymers of 2,6-dimethylphenol with other phenols (for example, 2,3,6-trimethylphenol and 2-methyl-6-butylphenol). Among these, poly(2,6-dimethyl-1,4-phenylene ether) and a copolymer of 2,6-dimethylphenol with 2,3,6-trimethylphenol are preferable, and poly(2,6-dimethyl-1,4-phenylene ether) is especially preferable.

A method for producing a PPE resin is not especially limited; and a PPE resin can easily be produced, for example, by oxidatively polymerizing 2,6-xylenol with the use of a complex of a cuprous copper salt with an amine as a catalyst, as described in U.S. Pat. No. 3,306,874. PPE resins can easily be produced also by methods described in U.S. Pat. Nos. 3,306,875, 3,257,357, and 3,257,358, Japanese Patent Publication No. 52-17880, Japanese Patent Laid-Open Nos. 50-51197 and 63-152628, and the like.

In addition to a single PPE described above, also a PPE resin can suitably be used which is prepared by blending a polystyrene (including an atactic high-impact polystyrene) having atactic or syndiotactic stereoregularity in the range of 1 to 400 parts by mass with respect to 100 parts by mass of the PPE component described above.

Also PPE resins can suitably be used which are prepared by incorporating reactive functional groups to various types of PPE described above. The reactive functional groups include an epoxy group, an oxazonyl group, an amino group, an isocyanate group, a carbodiimide group, and other acidic functional groups; and among these, acidic functional groups are more suitably used. An acidic functional group to be incorporated is not especially limited, but preferable are a sulfonic acid group, a phosphoric acid group, a carboxylic acid group, a maleic acid group, a maleic anhydride group, a fumaric acid group, an itaconic acid group, an acrylic acid group, and a methacrylic acid group; and especially preferable is a sulfonic acid group.

The weight-average molecular weight of a PPE resin is preferably 1,000 or higher and 5,000,000 or lower, and more preferably 1,500 or higher and 1,000,000 or lower. Here, the weight-average molecular weight refers to a value measured by gel permeation chromatography (GPC).

In the present embodiment 4, since each member constituting a redox flow secondary battery and an electrolyte membrane, and physical properties thereof are similar to those in the present embodiment 1, the description thereof will be omitted.

The electrolyte membranes in the present embodiments 1 to 4 are excellent in the ion permselectivity, low in the electric resistance, and excellent also in the durability (mainly the hydroxy radical oxidation resistance), and exhibit excellent performance as a separation membrane for a redox flow secondary battery. Here, each physical property in the present specification can be measured according to methods described in the following Examples unless otherwise specified.

EXAMPLES

Then, the present embodiments will be described more specifically by way of Examples and Comparative Examples, but the present embodiments are not limited to the following Examples unless going over their gist.

[Measurement Methods]

(1) The Melt Flow Index of a PFSA Resin Precursor

The melt flow index was measured according to ASTM: D1238 under the measurement conditions of a temperature of 270° C. and a load of 2,160 g.

(2) The Measurement of an Equivalent Weight EW of a PFSA Resin

0.3 g of a PFSA resin was immersed in 30 mL of a saturated NaCl aqueous solution at 25° C., and left for 30 min under stirring. Then, free protons in the saturated NaCl aqueous solution was subjected to a neutralization titration using a 0.01 N sodium hydroxide aqueous solution with phenolphthalein as an indicator. The end point of the neutralization titration was set at a pH of 7; and the PFSA resin content, obtained after the neutralization titration, in which counter ions of ion-exchange groups were in the sodium ion state was rinsed with pure water, further dried in a pan drier at 160° C., and weighed. The amount of substance of sodium hydroxide used for the neutralization was taken as M (mmol), and the mass of the PFSA resin in which counter ions of the ion-exchange groups were in the sodium ion state was taken as W (mg); and the equivalent weight EW (g/eq) was determined from the following expression. EW=(W/M)−22

The above operation was repeated five times; and the maximum value and the minimum value of the five calculated EW values were excluded, and the three values were arithmetically averaged to thereby make a measurement result.

(3) Membrane Thickness

An electrolyte membrane was allowed to stand still in a 23° C., 50% RH constant-temperature constant-humidity chamber for 1 hour or longer, and thereafter, the membrane thickness was measured using a film thickness meter (made by Toyo Seki Seisaku-sho, Ltd., trade name: “B-1”).

(4) the Measurements of an Equilibrium Moisture Content and a Maximum Moisture Content

A dispersion liquid of a PFSA resin was coated on a clear glass plate, dried at 150° C. for about 10 min, and peeled to thereby form a membrane of about 30 μm; the membrane was left in water at 23° C. for about 3 hours, and thereafter left in a room of a relative humidity (RH) of 50% for 24 hours; and then, the equilibrium moisture content was measured. An 80° C.-vacuum-dried membrane was used as the reference dried membrane. The equilibrium moisture content was calculated from the mass variation in the membrane. A maximum value observed in the equilibrium moisture content measurement was taken as the maximum moisture content.

(5) the Measurement of an Amount of Fluorine Ions Eluted

A PFSA resin was held in a glove box in which nitrogen was made to flow for 24 hours; about 0.1 g of the PFSA resin was weighed in the glove box; within 1 min after 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions was prepared, the weighed PFSA resin was immersed in the Fenton's reagent solution at 40° C. for 16 hours under no stirring. A solution mass was measured after the membrane was removed; and the fluorine ion concentration in the solution was measured by an ion meter, and the amount of the fluorine ions eluted was calculated.

(6) Charge and Discharge Test

In a redox flow secondary battery, each of liquid-permeable porous current collector electrodes (for a negative electrode and for a positive electrode) was disposed on either side of the separation membrane, and these were held by pressing; one side partitioned by the separation membrane was made a positive electrode cell chamber and the other side was made a negative electrode cell chamber; and the thicknesses of both the cell chambers were secured by spacers. Charge and discharge of the battery was carried out by circulating a positive electrode electrolyte solution composed of a sulfuric acid electrolyte solution comprising tetravalent vanadium (V⁴⁺) and pentavalent vanadium (V⁵⁺) to the positive electrode cell chamber, and circulating a negative electrode electrolyte solution comprising trivalent vanadium (V³⁺) and divalent vanadium (V²⁺) to the negative electrode cell chamber. In the charge time therein, in the positive electrode cell chamber, vanadium ions released electrons to thereby oxidize V⁴⁺ to V⁵⁺; and in the negative electrode cell chamber, electrons having returned through an external circuit reduced V³⁺ to V²⁺. In the oxidation and reduction reactions, in the positive electrode cell chamber, protons (H⁺) became excessive; by contrast, in the negative electrode cell chamber, protons (H⁺) became insufficient. The excessive protons in the positive electrode cell chamber selectively migrated to the negative electrode chamber through the separation membrane to thereby hold the electric neutrality. In the discharge time, a reaction reverse thereto progressed. The battery efficiency (energy efficiency) (%) at this time is represented by a ratio (%) obtained by dividing a discharge electric energy by a charge electric energy;

and both the electric energies depend on the internal resistance of the battery cells, the ion permselectivity of the separation membrane, and the current losses of others. The current efficiency (%) is represented by a ratio (%) obtained by dividing an amount of discharge electricity by an amount of charge electricity; and both the amounts of electricity depend on the ion permselectivity of the separation membrane and current losses of others. The battery efficiency is represented by a product of the current efficiency and a voltage efficiency. Since the reduction of the internal resistance, that is, cell electric resistivity, improves the battery efficiency (energy efficiency) and the improvement of the ion permselectivity and the reduction of the current losses of others improve the current efficiency, these factors become important indices in the redox flow secondary battery.

A charge and discharge test was carried out using a battery thus obtained. An aqueous electrolyte solution having a whole vanadium concentration of 2 M/L and a whole sulfate ion concentration of 4 M/L was used; the thicknesses of the positive electrode cell chamber and the negative electrode cell chamber installed were each 5 mm; and a porous felt of 5 mm in thickness and about 0.1 g/cm³ in bulk density composed of a carbon fiber was interposed between the separation membrane and each of both the porous electrodes. The charge and discharge test was carried out at a current density of 80 mA/cm².

The cell electric resistivity (Ω·cm²) was determined by using the AC impedance method, and measuring a direct-current resistance value at an AC voltage of 10 mV at a frequency of 20 kHz at the discharge initiation time and multiplying the resistance value by the electrode area.

(7) Durability

The durability was evaluated using the current efficiency (%) and the cell electric resistivity (Ω·cm²) after 200 cycles of the charge and discharge of the above (6) were carried out.

Example 1 (1) Production of a PFSA Resin Precursor

A 10% aqueous solution of C₇F₁₅COONH₄ and pure water were charged in a stainless steel-made stirring-type autoclave, and the interior atmosphere of the autoclave was sufficiently replaced by vacuum and nitrogen; and thereafter, tetrafluoroethylene (CF₂═CF₂) (hereinafter, also referred to as “TFE”) gas was introduced, and the interior pressure was pressurize to 0.7 MPa in terms of gage pressure. Then, an ammonium persulfuric acid aqueous solution was injected to initiate the polymerization. While in order to supply TFE consumed by the polymerization, TFE gas was continuously fed so as to hold the pressure of the autoclave at 0.7 MPa, CF₂═CFO(CF₂)₂—SO₂F of an amount corresponding to 0.70 times the amount of TFE fed in mass ratio was continuously fed to carry out the polymerization by regulating the polymerization condition in a best range to thereby obtain a perfluorocarbonsulfonic acid resin precursor powder. The MFI of the obtained PFSA resin precursor powder A1 was 1.5 (g/10 min).

(2) Fluorination Treatment of Molecular Chain Terminals

A multistage tray fabricated of a Hastelloy C alloy was placed in a pressure-resistant reaction vessel of 50 L in interior volume whose inner surface was fabricated of a Hastelloy C alloy; and a mixed gas of 20% of fluorine gas and 80% of nitrogen gas was introduced at 0.25 MPa in gage pressure, and the temperature was held at 190° C. for 4 hours to thereby subject the metal surface to a passivation treatment. The temperature was decreased, and thereafter, the above-mentioned PFSA resin precursor powder was put in the 50-L pressure-resistant vessel; and a mixed gas of 20% of fluorine gas and 80% of nitrogen gas was introduced at 0.25 MPa in gage pressure, and the temperature was held at 180° C. for 4 hours to thereby subject the powder to a fluorination treatment. After the fluorination treatment, the fluorine gas was exhausted; and a polymer was taken out, and crushed by a crusher to thereby obtain a fluorinated polymer (hereinafter, also referred to as “precursor polymer”) having —SO₂F groups being precursor groups of sulfonic acid groups.

(3) Production of PFSA Acid Resins and Dispersion Solutions Thereof

The obtained precursor polymer was brought into contact with an aqueous solution in which potassium hydroxide (15% by mass) and methyl alcohol (50% by mass) were dissolved at 80° C. for 20 hours to thereby subject the precursor polymer to a hydrolysis treatment. Thereafter, the precursor polymer was immersed in water at 60° C. for 5 hours. Then, such a treatment that the resultant was immersed in a 2 N hydrochloric acid aqueous solution at 60° C. for 1 hour was repeated five times by renewing the hydrochloric acid aqueous solution each time; and thereafter, the resultant was washed with ion-exchange water, and dried. A PFSA resin having a structure having sulfonic acid groups (SO₃H) and represented by the formula (2) (m=2) was thereby obtained. The EW of the obtained PFSA resin A1 was 650 (g/eq). The EW of a PFSA resin A2 obtained using the PFSA resin precursor power A1 instead of the precursor polymer was 650 (g/eq).

The obtained PFSA resins A1 and A2 were each put in a 5-L autoclave together with an ethanol aqueous solution (water:ethanol=50:50 (in mass ratio)), and the autoclave was hermetically closed; and the mixture was heated up to 160° C. under stirring by a blade, and the temperature was held for 5 hours. Thereafter, the autoclave was spontaneously cooled, and a homogeneous dispersion liquid of 5% by mass of the PFSA resin was thus fabricated. Then, 100 g of pure water was added to 100 g of the PFSA resin dispersion liquid, and stirred; and thereafter while the dispersion liquid was heated to 80° C. and stirred, the dispersion liquid was concentrated up to 20% by mass in terms of solid content concentration.

The PFSA resin dispersion liquids obtained from the PFSA resins A1 and A2 were named dispersion liquids (ASF1) and (ASF2), respectively.

(4) Production of an Electrolyte Membrane

The obtained dispersion liquid (ASF1) was cast on a polyimide film as a carrier sheet by a well-known usual method, exposed to hot air at 120° C. (for 20 min) to nearly completely evaporate the solvent and dry to thereby obtain a membrane. The membrane was further subjected to a heat treatment in a hot air atmosphere under the condition of 160° C. for 10 min to thereby obtain an electrolyte membrane of 50 μm in membrane thickness. The variation rate of EWs before and after the heat treatment of the obtained electrolyte membrane was about 0.2 to 0.3%.

The equilibrium moisture content of the obtained electrolyte membrane was 12% by mass for ASF1.

The maximum moisture content of the electrolyte membrane in water at 25° C. for 3 hours was 23% by mass for ASF1.

Then, a charge and discharge test was carried out using the obtained electrolyte membrane as a separation membrane of a vanadium redox flow secondary battery. The charge and discharge test was carried out by using the membrane obtained from ASF1 and after the equilibrium was sufficiently reached in the electrolyte solution; and thereafter, after the stable state was made, the cell electric resistivity and the current efficiency were measured; the current efficiency/the cell electric resistivity was (97.5/0.90) for the membrane of ASF1, thus giving an excellent tendency.

Then, a durability test was carried out by using the membrane obtained from ASF1 and carrying out 200 cycles of the charge and discharge and examining the variation. As a result, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was (97.3/0.90) for ASF1, giving only a small variation and exhibiting excellent oxidation resistance.

The amount of the fluorine ions eluted was measured by using the obtained polymer, and was 0.001% of the whole amount of fluorine in the immersed polymer.

Example 2

An electrolyte membrane was obtained by the similar method as in Example 1, except for using Nafion DE2021CS (registered trademark, made by Du Pont K.K., 20% solution) instead of the dispersion liquid (ASF1). The equilibrium moisture content of the membrane was 6% by mass, and the maximum moisture content was 14% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 94.5/1.20; and as a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 94%, and the electric resistance was 1.20.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.002% of the whole amount of fluorine in the immersed polymer.

Example 3

An electrolyte membrane of 50 μm in membrane thickness obtained by the similar method as in Example 1 by using the dispersion liquid (ASF2) was immersed in a 1% cerium nitrate aqueous solution in which cerium nitrate was dissolved in distilled water, and stirred by a stirrer at room temperature for 40 hours to thereby incorporate cerium ions in the electrolyte membrane. As a result of analyzing the cerium nitrate solution before and after the immersion by inductively coupled plasma (ICP) atomic emission spectrometry, the content of cerium ions in the electrolyte membrane (a proportion of cerium ions with respect to the number of —SO₃— groups in the membrane) was 10.2%.

The equilibrium moisture content of the obtained electrolyte membrane was 12% by mass, and the maximum moisture content was 23% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 97.5/0.90; and as a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 97.3/0.90, giving so small a variation, and exhibiting excellent oxidation resistance.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.008% of the whole amount of fluorine in the immersed polymer.

Example 4

An electrolyte membrane was obtained by the similar method as in Example 3, except for using Nafion DE2021 (registered trademark, made by Du Pont K.K., 20% solution, EW: 1,050) instead of the dispersion liquid (ASF2) and using cerium carbonate instead of cerium nitrate. The equilibrium moisture content of the membrane was 6% by mass, and the maximum moisture content was 14% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 94.5/1.20, in which the current efficiency was in a lower level than in Example 3. This is presumably because the electrolyte membrane of Example 4 had a slightly low ion permselectivity. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 94% and the electric resistance was 1.20, also exhibiting slightly inferior durability.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.01% of the whole amount of fluorine in the immersed polymer.

Example 5 and 6

An electrolyte membrane of 50 μm in membrane thickness obtained by the similar method as in Example 1 by using the dispersion liquid (ASF2) was immersed in a 1% aqueous solution in which cobalt nitrate (Example 5) or manganese acetate (Example 6) was dissolved in distilled water, and stirred by a stirrer at room temperature for 40 hours to thereby incorporate cobalt ions or manganese ions in the electrolyte membrane. As a result of analyzing the cobalt nitrate solution or the manganese acetate solution before and after the immersion by inductively coupled plasma (ICP) atomic emission spectrometry, the content of cobalt ions or manganese ions in the electrolyte membrane (a proportion of cobalt ions or manganese ions with respect to the number of —SO₃— groups in the corresponding membrane) was 14.7% or 13.8%, respectively.

The equilibrium moisture content of the obtained electrolyte membrane was 12% by mass for either membrane of Examples 5 and 6, and the maximum moisture content was 23% by mass for either.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 97.5/0.90 for either; and as a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 97.3/0.90, giving so small a variation, and exhibiting excellent oxidation resistance.

The amounts of the fluorine ions eluted were measured by using the obtained electrolyte membranes, and were 0.01% of the whole amounts of fluorine in the immersed polymers.

Examples 7 and 8

Electrolyte membranes were obtained by the similar methods as in Examples 5 and 6, except for using Nafion DE2021 (made by Du Pont K.K., 20% solution, EW: 1,050) instead of the dispersion liquid (ASF2). The equilibrium moisture contents of the membranes were 6% by mass for either of the electrolyte membranes of Examples 7 and 8, and the maximum moisture contents were 14% by mass for either.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 94.5/1.20 for either, in which the current efficiencies were in a lower level than in Examples 5 and 6. This is presumably because the electrolyte membranes of Examples 7 and 8 had a slightly low ion permselectivity. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 94% and the electric resistance was 1.20 for either, also exhibiting slightly inferior durability.

The amounts of the fluorine ions eluted were measured by using the obtained electrolyte membranes, and were 0.02% of the whole amounts of fluorine in the immersed polymers.

Example 9

A PPS powder (made by Chevron Phillips Chemical Co. LP, type No. P-4) dissolved in an alkali aqueous solution (KOH-10% aqueous solution) was homogeneously mixed and dispersed under stirring in the PFSA resin dispersion liquid (ASF2) so that the PPS powder was finally (in terms of solid component) 5 parts by mass with respect to 100 parts by mass of the PFSA resin component. Then, the resultant was passed through a column packed with a particulate cation-exchange resin particle to thereby nearly completely remove alkali ion components, to thereby make a mixed dispersion liquid (ASF3) in which ionic bonds of at least a part of the functional groups (sulfonic acid groups and alkaline nitrogen atoms) were generated.

The obtained mixed dispersion liquid was cast on a polyimide film as a carrier sheet by a well-known usual method, exposed to hot air at 120° C. (for 20 min) to nearly completely evaporate the solvent and dry to thereby obtain a membrane. The membrane was further subjected to a heat treatment in a hot air atmosphere under the condition of 160° C. for 10 min to thereby obtain an electrolyte membrane of 25 μm in membrane thickness. The variation rate of EWs before and after the heat treatment of the obtained electrolyte membrane was about 0.2 to 0.3%. The equilibrium moisture content of the obtained electrolyte membrane was 12% by mass.

The maximum moisture content of the electrolyte membrane in water at 25° C. for 3 hours was 18% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 98.2/0.95; and as a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 98/0.95, giving so small a variation, and exhibiting excellent oxidation resistance.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.01% of the whole amount of fluorine in the immersed polymer.

Example 10

An electrolyte membrane was obtained by the similar method as in Example 9, except for using Nafion DE2021 (made by Du Pont K.K., 20% solution, EW: 1,050) instead of the dispersion liquid (ASF2). The equilibrium moisture content of the membrane was 6% by mass, and the maximum moisture content was 14% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 97.2/0.98. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 97% and the cell electric resistance was 0.99 Ω·cm².

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.02% of the whole amount of fluorine in the immersed polymer.

Example 11

An electrolyte membrane was obtained by the similar method as in Example 9, except for using a PPE powder (made by Nihon Extron Co., Ltd.) instead of the PPS powder used in Example 9. The equilibrium moisture content of the membrane was 11% by mass, and the maximum moisture content was 18% by mass. As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 98/0.95. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 97.8% and the cell electric resistance was 0.95 Ω·cm².

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.01% of the whole amount of fluorine in the immersed polymer.

Comparative Example 1

An electrolyte membrane was obtained by the similar method as in Example 1, except for using Nafion DE2021 (made by Du Pont K.K., 20% solution, EW: 1,050) instead of the dispersion liquid (ASF1). The equilibrium moisture content of the membrane was 4% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 94.5/1.20, in which the current efficiency was in a lower level than in Examples 1 and 2. This is presumably because the electrolyte membrane of Comparative Example 1 had a low ion permselectivity. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 86.0% and the electric resistance was 1.30, also exhibiting inferior durability.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.05% of the whole amount of fluorine in the immersed polymer.

Comparative Example 2

An electrolyte membrane was obtained by the similar method as in Example 1, except for using the dispersion liquid (ASF2) instead of the dispersion liquid (ASF1). The equilibrium moisture content of the membrane was 12% by mass, and the maximum moisture content was 23% by mass.

As a result of carrying out a charge and discharge test by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 97.5/0.90. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 89.7% and the electric resistance was 1.18, exhibiting inferior durability.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane, and was 0.04% of the whole amount of fluorine in the immersed polymer.

Comparative Example 3

As a result of carrying out a charge and discharge test by using Nafion 112 (made by Du Pont K.K., membrane thickness: 50 μm) by the similar method as in Example 1, the current efficiency (%)/the cell electric resistivity (Ω·cm²) was 94.0/1.20. As a result of carrying out 200 cycles of the charge and discharge as the durability test, the current efficiency was 85.2% and the electric resistance was 1.30, exhibiting inferior durability.

The amount of the fluorine ions eluted was measured by using the obtained electrolyte membrane by the similar method as in Example 1, and was 0.05% of the whole amount of fluorine in the immersed polymer.

In Table 1, the results of the above Examples 1 to 11 and Comparative Examples 1 to 3 are shown.

TABLE 1 Polyelectrolyte Polyelectrolyte Polymer Membrane Equivalent Additive Membrane Polymer Weight Terminal Content Composition Thickness Name MFI (g/10 min) (g/eq) Fluorination Kind (%) Name (μm) Example 1 A1 1.5 650 present — — ASF1 50 Example 2 Nafion — 1050 present — — — 50 DE2021CS Example 3 A1 1.5 650 absent cerium 10 ASF2 50 nitrate Example 4 Nafion — 1050 absent cerium 10 — 50 DE2021 carbonate Example 5 A1 1.5 650 absent cobalt nitrate 14.7 ASF2 50 Example 6 A1 1.5 650 absent manganese 13.8 ASF2 50 acetate Example 7 Nafion — 1050 absent cobalt nitrate 25 — 50 DE2021 Example 8 Nafion — 1050 absent manganese 25 — 50 DE2021 acetate Example 9 A1 1.5 650 absent PPS 5 ASF2 25 Example 10 Nafion — 1050 absent PPS 5 — 25 DE2021 Example 11 A1 1.5 650 absent PPE 5 ASF2 25 Comparative Nafion — 1050 absent — — — 50 Example 1 DE2021 Comparative A1 1.5 650 absent — — ASF2 50 Example 2 Comparative — — — — — — Nafion 50 Example 3 112 Polyelectrolyte Membrane Membrane Change and Discharge Test of Redox Flow Battery Membrane Maximum Current Cell Electric Equilibrium Moisture Current Cell Electric Efficiency Resistivity Moisture Content Amount of Membrane Efficiency Resistivity (%) (Ω · cm²) Content (25° C.) Fluorine Ions Thickness (%) (Ω · cm²) (after 200 (after 200 (mass %) (mass %) Eluted (%) (μm) (initial) (initial) cycles) cycles) Example 1 12 23 0.001 50 97.5 0.90 97.3 0.90 Example 2 6 14 0.002 50 94.5 1.20 94.0 1.20 Example 3 12 23 0.008 50 97.5 0.90 97.3 0.90 Example 4 6 14 0.01 50 94.5 1.20 94.0 1.20 Example 5 12 23 0.01 50 97.5 0.90 97.3 0.90 Example 6 12 23 0.01 50 97.5 0.90 97.3 0.90 Example 7 6 14 0.02 50 94.5 1.20 94.0 1.20 Example 8 6 14 0.02 50 94.5 1.20 94.0 1.20 Example 9 12 18 0.01 25 98.2 0.95 98.0 0.95 Example 10 6 14 0.02 25 97.2 0.98 97.0 0.99 Example 11 11 18 0.01 25 98.0 0.95 97.8 0.95 Comparative 4 — 0.05 50 94.5 1.20 86.0 1.30 Example 1 Comparative 12 23 0.04 50 97.5 0.90 89.7 1.18 Example 2 Comparative — — 0.05 50 94.0 1.20 85.2 1.30 Example 3

The present application is based on Japanese Patent Applications (Japanese Patent Application Nos. 2011-290097, 2011-290070, and 2011-290077), filed on Dec. 28, 2011 in the Japan Patent Office, and a Japanese Patent Application (Japanese Patent Application No. 2012-010245), filed on Jan. 20, 2012 in the Japan Patent Office, the entire content of which are hereby incorporated by reference.

INDUSTRIAL APPLICABILITY

The redox flow secondary battery according to the present invention is low in the electric resistance, high in the current efficiency, and excellent in the durability as well.

The electrolyte membrane for a redox flow secondary battery according to the present invention is excellent in the ion permselectivity, low in the electric resistance, and excellent in the durability (mainly hydroxy radical oxidation resistance) as well, and is industrially applicable as a separation membrane for a redox flow secondary battery.

REFERENCE SIGNS LIST

-   1 POSITIVE ELECTRODE -   2 POSITIVE ELECTRODE CELL CHAMBER -   3 NEGATIVE ELECTRODE -   4 NEGATIVE ELECTRODE CELL CHAMBER -   5 ELECTROLYTE MEMBRANE -   6 ELECTROLYTIC BATH -   7 POSITIVE ELECTRODE ELECTROLYTE SOLUTION TANK -   8 NEGATIVE ELECTRODE ELECTROLYTE SOLUTION TANK -   9 AC/DC CONVERTER -   10 REDOX FLOW SECONDARY BATTERY 

The invention claimed is:
 1. An electrolyte membrane for a redox flow secondary battery, comprising an ion-exchange resin composition comprising a fluorine-based polyelectrolyte polymer having a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents SO₃Z, wherein Z represents a hydrogen atom; R¹ and R² each independently represents one or more selected from the group consisting of halogen atoms, perfluoroalkyl groups having 1 to 10 carbon atoms, and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); and wherein the fluorine-based polyelectrolyte polymer has a terminal group of a main molecular chain of CF₃; in a test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, an amount of fluorine ions eluted detected in a solution is 0.03% or smaller of a whole amount of fluorine in an immersed polymer.
 2. The electrolyte membrane for a redox flow secondary battery according to claim 1, wherein in the test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of the Fenton's reagent solution containing the 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, the amount of fluorine ions eluted detected in the solution is 0.002% or smaller of the whole amount of fluorine in the immersed polymer.
 3. The electrolyte membrane for a redox flow secondary battery according to claim 1, wherein the fluorine-based polyelectrolyte polymer is a perfluorocarbonsulfonic acid resin (PFSA) having a structure represented by the following formula (2): —[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1; and m represents an integer of 1 to
 6. 4. The electrolyte membrane for a redox flow secondary battery according to claim 1, wherein the fluorine-based polyelectrolyte polymer has an equivalent weight EW (dry mass in grams per equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and the electrolyte membrane has an equilibrium moisture content of 5 to 80% by mass.
 5. The electrolyte membrane for a redox flow secondary battery according to claim 1, wherein the ion-exchange resin composition comprises 0.1 to 20 parts by mass of a polyphenylene ether resin and/or a polyphenylene sulfide resin with respect to 100 parts by mass of the fluorine-based polyelectrolyte polymer.
 6. The electrolyte membrane for a redox flow secondary battery according to claim 5, wherein the fluorine-based polyelectrolyte polymer is a perfluorocarbonsulfonic acid resin (PFSA) having a structure represented by the following formula (2): —[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1; and m represents an integer of 1 to
 6. 7. The electrolyte membrane for a redox flow secondary battery according to claim 5, wherein the fluorine-based polyelectrolyte polymer has an equivalent weight EW (dry mass in grams per equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and the electrolyte membrane has an equilibrium moisture content of 5 to 80% by mass.
 8. The electrolyte membrane for a redox flow secondary battery according to claim 1, wherein the ion-exchange resin composition comprises one or more selected from the group consisting of a Ce-based additive, a Co-based additive, and a Mn-based additive.
 9. The electrolyte membrane for a redox flow secondary battery according to claim 8, wherein the fluorine-based polyelectrolyte polymer is a perfluorocarbonsulfonic acid resin (PFSA) having a structure represented by the following formula (2): —[CF₂—CF₂]_(a)—[CF₂—CF((—O—(CF₂)_(m)—SO₃H)]_(g)—  (2) wherein a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1; and m represents an integer of 1 to
 6. 10. The electrolyte membrane for a redox flow secondary battery according to claim 8, wherein the fluorine-based polyelectrolyte polymer has an equivalent weight EW (dry mass in grams per equivalent of ion-exchange groups) of 300 to 1,300 g/eq; and the electrolyte membrane has an equilibrium moisture content of 5 to 80% by mass.
 11. A redox flow secondary battery comprising an electrolytic bath comprising: a positive electrode cell chamber comprising a positive electrode composed of a carbon electrode; a negative electrode cell chamber comprising a negative electrode composed of a carbon electrode; and electrolyte membrane according to claim 1 as a separation membrane to separate the positive electrode cell chamber and the negative electrode cell chamber.
 12. The redox flow secondary battery according to claim 11, wherein sulfuric acid electrolyte solutions comprising vanadium are used as the positive electrode electrolyte solution and the negative electrode electrolyte solution.
 13. The redox flow secondary battery according to claim 11, wherein the ion-exchange resin composition comprises 0.1 to 20 parts by mass of a polyphenylene ether resin and/or a polyphenylene sulfide resin with respect to 100 parts by mass of the fluorine-based polyelectrolyte polymer.
 14. The redox flow secondary battery according to claim 13, wherein sulfuric acid electrolyte solutions comprising vanadium are used as the positive electrode electrolyte solution and the negative electrode electrolyte solution.
 15. The redox flow secondary battery according to claim 11, wherein the ion-exchange resin composition comprises one or more selected from the group consisting of a Ce-based additive, a Co-based additive, and a Mn-based additive.
 16. The redox flow secondary battery according to claim 15, wherein sulfuric acid electrolyte solutions comprising vanadium are used as the positive electrode electrolyte solution and the negative electrode electrolyte solution.
 17. A method of producing the electrolyte membrane for a redox flow secondary battery according to claim 1, comprising: fluorinating a precursor of fluorine-based polyelectrolyte polymer, and hydrolyzing and acidizing the fluorinated precursor into a fluorine-based polyelectrolyte polymer, and then obtaining the electrolyte membrane using a cast membrane formation method with the fluorine-based polyelectrolyte polymer, or extrusion-mixing the fluorinated precursor, and then obtaining the electrolyte membrane of a fluorine-based polyelectrolyte polymer with a hydrolysis treatment and an acid treatment on the extrusion-mixed fluorinated precursor, wherein a terminal group of a side molecular chain of the precursor contains SO₂F; wherein the fluorine-based polyelectrolyte polymer has a terminal group of a main molecular chain of CF₃; wherein, the fluorine-based polyelectrolyte polymer has a structure represented by the following formula (1): —[CF₂—CX¹X²]_(a)—[CF₂—CF((—O—CF₂—CF(CF₂X³))_(b)—O_(c)—(CFR¹)_(d)—(CFR²)_(e)—(CF₂)_(f)—X⁴)]_(g)—  (1) wherein X¹, X², and X³ each independently represent one or more selected from the group consisting of halogen atoms and perfluoroalkyl groups having 1 to 3 carbon atoms; X⁴ represents SO₃Z, wherein Z represents a hydrogen atom; R¹ and R² each independently represents one or more selected from the group consisting of halogen atoms, perfluoroalkyl groups having 1 to 10 carbon atoms, and fluorochloroalkyl groups having 1 to 10 carbon atoms; and a and g represent numbers satisfying 0≤a<1, 0<g≤1, and a+g=1, b represents an integer of 0 to 8, c represents 0 or 1, and d, e, and f each independently represent an integer of 0 to 6 (with the proviso that d, e, and f are not 0 at the same time); wherein in a test in which 0.1 g of the fluorine-based polyelectrolyte polymer is immersed in 50 g of a Fenton's reagent solution containing a 3% hydrogen peroxide solution and 200 ppm of divalent iron ions at 40° C. for 16 hours, an amount of fluorine ions eluted detected in a solution is 0.03% or smaller of a whole amount of fluorine in an immersed polymer. 